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P0300 code (random misfire)

18K views 31 replies 7 participants last post by  Big Blue 
#1 ·
Searching through forum after forum, thread after thread but not sure what to do.


First, I have a 2008 Commander Sport 3.7L with 103k miles.


Ive been wracking my brain trying to figure out this issue. A few months ago, my TPS, Airbag & Check Engine light were all on. I was able to get the airbag light to go out by checking/cleaning all connections under the seats and replaced the fuses which seemed a bit corroded. All air pressure is fine so I assume the TPS light is due to a faulty TPS valve.


I am getting a P0300 code (random misfire) but no cylinder code. I'll drive the Jeep and it will be fine. I'll go to drive it again shortly afterwards it will through the check engine light and run really rough. Lately it now seems to be sluggish and then all of a sudden the power will kick in. Someone posted on another forum that he noticed it seemed to shift between gears alot especially going up hills. Never thought much about it but mine does as well. I usually manually downshift it a gear to keep some power but driving through the hills of Vermont last year I really noticed it had no power. Sometimes I will also smell gasoline but it goes away.


Things I have done:
- Put new NGK Iridium spark plugs
- Replaced all coil packs

- Tested the battery
- Replaced a broken ground strap ( the one that attaches to the exhaust, frame and transmission,)
- Cleaned electrical connections
- Replaced transmission coolant lines and had a dealer check the fluid level
- Recently changed out the Iridium plugs for the original copper plugs. In doing so, I noticed I didnt tighten the plugs down as much as I should have. I blew compressed air into the holes to remove any dirt as recommended by the service manual. When I did that to cylinder 3, some oil came splashing back up at me.


- Known fixes, but not sure where to start:


- Air Intake sensor
- Mass Air meter sensor
- Throttle position sensor
- Crankshaft position sensor
- Camshaft position sensor
- Oxygen sensors
- Fuel pressure
- Vacuum leak
- Fuel injectors
- New PCM

- And so on.....


I didnt know if someone should suggest where to start since its semi intermittent and seems to be more when Ive driven it and it starts when i go to drive it again shortly afterwards.



Confused and fed up with this issue.
 
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#2 · (Edited)
@msnowdon; I can't help you much with the P0300 code short of advising you to go to a shop or dealer with a real OBD-II scanner and have all the DTC's pulled along with the information that comes with them and see what that tells you; This thread my point you in the right direction;

https://www.jeepcommander.com/forums/58-diagnostic-trouble-codes-dtc/43218-p0300-random-misfire.html

The TPM sensor can be triggered by more than just low tire pressure; the individual TPM sensors all have their own self-contained batteries and if the battery dies, that sensor can no longer check in with the TPM system and will trigger that annoying orange light to go off; I honestly don't know off-hand if the batteries can be replaced or not; If not, the TPM sensors themselves are pretty cheap and you can pick them up off of amazon or e-bay, I believe Doorman is the company that makes them and there are probably others. You also have a TPM sensor in the spare tire - which a lot of people seem to overlook when trying to troubleshoot their TPM sensor system.

As far as the constant downshifting, that's probably due to your Hill Start Assist / Hill Descent Control assuming that you have this feature and I believe almost all of the different XK trim packages do regardless of the power-train you have; If you question whether you have this feature or not, it would be listed on your factory build sheet - if you don't have one, I can tell you how to get one, it's easy enough. If your Hill Start Assist is on, you will tend to downshift much more often, particularly if you are towing under load. You can do away with the excessive down-shifting by just turning it off; If you go to your EVIC and toggle through all of your personal settings, when you come to the correct sub-menu, you will have an option to just turn it on or off by pressing the buttons on your steering wheel like you would do to change any other setting in your personal settings menu.
 
#3 ·
TPS and airbag lights are typical on these Jeeps, and I wouldn't be concerned about those at all. Even just a water bottle rolling under a seat and touching an airbag harness is enough to set the airbag light off.

The P0300, rough running, lack of power, and downshifting all are consistent with an engine problem, though I'm sure your transmission service didn't hurt. An intermittent misfire typically does lead to a gasoline scent, so that's consistent as well.

Unfortunately, a P0300 is tough to diagnose; engines don't typically have a way of knowing whether combustion is happening in a given cylinder apart from feedback from the spark plug and injector. This means that you could be having a misfire in a specific cylinder, but the computer has no way of knowing which cylinder so it throws a P0300.

I'd start by running an injector cleaner through your gas tank and looking for a vacuum leak if you've got the tools. Obviously you've got spark sorted out, and typically I'd expect to see more codes if a sensor was having problems, so I'd start with the basics and make sure you've got good fuel and air supplies. You can also read the spark plugs to see if there's an obvious problem cylinder; given the oil in cylinder 3, you may want to start there.
 
#4 ·
A couple of more questions:


If I disconnect the harness to the mass air intake sensor while it is running, wouldnt I notice a change in the idle? It made no effect what so ever.


Also, is checking the battery voltage with a multi-meter tool enough or is there a more extensive test? Weird thing I noticed is that I get corrosion on the positive post only. Never the negative post.


Thanks


Mark
 
#5 · (Edited)
A couple of more questions:


If I disconnect the harness to the mass air intake sensor while it is running, wouldnt I notice a change in the idle? It made no effect what so ever.


Also, is checking the battery voltage with a multi-meter tool enough or is there a more extensive test? Weird thing I noticed is that I get corrosion on the positive post only. Never the negative post.


Thanks


Mark
You can drive to any Auto-Zone or Advance Auto Parts and they will test your battery for free.
 
#6 ·
"As far as the constant downshifting, that's probably due to your Hill Start Assist / Hill Descent Control assuming that you have this feature and I believe almost all of the different XK trim packages do regardless of the power-train you have; If you question whether you have this feature or not, it would be listed on your factory build sheet -"


Hi Big Blue,


I went through the EVIC and Equipment Listing Sheet. My vehicle doesnt seem to have Hill Start Assist/Hill Descent Control. It does have Electronic Stability Control, Four Wheel Traction Control & Speed Control. Looks like Im one control short
 
#12 ·
I did actually. When I went back to the stock plugs, I pulled them out a week later, rechecked the gap and torqued them all. The gap is supposed to be .043" but I could not find any gauges that had .043" as a measurement. I went with the next closest which was .044" I dont like to use those round ramp style gauges.
 
#13 ·
So Im just about finished replacing my oxygen sensors. I just need to re-install the left front fender liner which I removed to access the wiring harness. Also ordered the scan tool and software that BigBlue was mentioning.



Should know this weekend if it helps my misfiring issue. Already replaced the plugs, ambient air sensor, IAT sensor, & MAP sensor.


Unfortunately I found a tear in the seal that connects my front drive shaft to the transfer case which is leaking grease and grease is leaking from my front shocks. Some refer to them as struts but from what Ive read, they are actually coil over shocks.
 
#14 ·
Unfortunately I found a tear in the seal that connects my front drive shaft to the transfer case which is leaking grease and grease is leaking from my front shocks. Some refer to them as struts but from what Ive read, they are actually coil over shocks.
They seem like struts to me. Struts are a structural part of a vehicle, shocks are not. Coil over shocks just give larger payload capacity. If you remove a shock the vehicle will remain at the same height, there is always some other spring for the wheel that has a shock . If you remove a strut the suspension will hard bottom out. It doesn't matter if the strut is componentized, it is installed as one unit. It's that single unit and the only spring for that wheel that makes it a strut and not a coil over shock with a coil on steroids.
 
#15 ·
I finally have had some time to drive the Commander after replacing the oxygen sensors, MAP sensor, Ambient Air sensor, IAT sensor, spark plugs & coil packs. I mean i did test drive it but my wife takes it most of the time and said it has been fine. I took it on the highway today and noticed the transmission wasnt kicking down correctly. It was acting weird when trying, like bogging down and causing the Check Engine light to come on.


When I got home I finally got to play with the Appcar DiagFCA software I purchased. Had no problem getting it installed and connected. Im just not used to having access to all this information. A stored DTC for a misfire on cylinder 3 showed up but wasnt active. Does that mean it didnt throw the light? That was the only DTC it picked up. I was getting confused with the Live Data feature. It has Misfire History Count for cylinders 1-4. What happened to 5 & 6? The software is up to date and shows as a 3.7 Jeep Commander. Also I know what clearing DTCs does but what exactly does Reset ECU ( I think thats the button, I dont have the software open in front of me.) Is that a good thing to try and when should you try it?


Going back to the Live Data in the Diagnostics tab. How does one know which options to check off? I tried checking all of them off for the PCM module just to see what I got for data but it was a pain. I didnt see a select all option, just the option to hide was isnt selected.


The "How To Use It" is okay for getting started, even with it's typos and "English is a Second Language" feel to it but it it doesnt dive into any details. Most of it is giving me the definition of the terms being used.


I do like the software and the programmable features are really cool. I just need to understand it better. I dont want to go screwing something up by accident. And Im still not sure if its misfiring or not. I did clear the stored DTC to see if anything is logged the next time it happens.



Going to play around with it again tomorrow. It was hot when i tried this afternoon and I was getting eaten alive by mosquitoes this evening.
 
#16 · (Edited)
I finally have had some time to drive the Commander after replacing the oxygen sensors, MAP sensor, Ambient Air sensor, IAT sensor, spark plugs & coil packs. I mean i did test drive it but my wife takes it most of the time and said it has been fine. I took it on the highway today and noticed the transmission wasnt kicking down correctly. It was acting weird when trying, like bogging down and causing the Check Engine light to come on.

When I got home I finally got to play with the Appcar DiagFCA software I purchased. Had no problem getting it installed and connected. Im just not used to having access to all this information. A stored DTC for a misfire on cylinder 3 showed up but wasnt active. Does that mean it didnt throw the light? That was the only DTC it picked up. I was getting confused with the Live Data feature. It has Misfire History Count for cylinders 1-4. What happened to 5 & 6? The software is up to date and shows as a 3.7 Jeep Commander. Also I know what clearing DTCs does but what exactly does Reset ECU ( I think thats the button, I dont have the software open in front of me.) Is that a good thing to try and when should you try it?

Going back to the Live Data in the Diagnostics tab. How does one know which options to check off? I tried checking all of them off for the PCM module just to see what I got for data but it was a pain. I didnt see a select all option, just the option to hide was isnt selected.

The "How To Use It" is okay for getting started, even with it's typos and "English is a Second Language" feel to it but it it doesnt dive into any details. Most of it is giving me the definition of the terms being used.

I do like the software and the programmable features are really cool. I just need to understand it better. I dont want to go screwing something up by accident. And Im still not sure if its misfiring or not. I did clear the stored DTC to see if anything is logged the next time it happens.

Going to play around with it again tomorrow. It was hot when i tried this afternoon and I was getting eaten alive by mosquitoes this evening.
Hey @msnowdon; Do you have any updates on this?

I'm curious as to what you did - or did not find......
 
#26 ·
So I ground down the caliper brackets and while I had the Jeep up, the wheels seemed to just clear them. I finished late and didnt get to test drive it until this morning. Going straight seemed fine but when I turn left, I hear a scraping noise from the front right and when I turn right, noise from the left. I think the wheels might be coming into contact with the knuckle? So I quickly threw my old ones back on i the front and the noise went away. So if I want to stay with the Wrangler wheels, Im going to need spacers. The Wrangler wheels look so much nicer and they were taken off a Jeep just after it was purchased so they are like new. Went with the metallic silver and satin black lug nuts and Pirrelli Scorpian All Terrain Plus. They look much more aggressive but not too aggressive. Also have the deep snow certification and great reviews. First time not going with Goodyears.



Again I dont know much about them. I dont want them sticking out too far. Looks like some of the thinner spacers use the existing wheel studs but may require longer ones.



Sevenjays, you are saying if I use 1" spacers, they mount to the existing studs and they must come with their own studs for mounting the wheel?


What are some of the better spacer manufactures? I know it has to 5x5.
 
#27 ·
Yes, my 1 1/2" use the existing studs to attach to the rotor, and have new studs installed in them to attach the rim.



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#28 ·



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#30 ·



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Sorry for the late response. Ive been busy with my Grand Cherokee and then I went on vacation. I ended up ordering 1.5" spacers made by SpiderTrax. The 1.5" came up as the recommended size for the Commander. I just installed them this past weekend. I love the look of it with the new tires and Wrangler Moab wheels. Looks more aggressive. I'll have to take some pictures.
 
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